Electroencephalographic and cardiovascular indicators of nociception during isoflurane anaesthesia in pigs

Objective To evaluate electroencephalographic indices, invasive arterial blood pressure and pulse rate as indicators of nociception during isoflurane anaesthesia in pigs. Animals Ten Norwegian Landrace pigs, five castrated males and five females, weighing between 19 and 29 kg. Materials and methods The pigs were anaesthetized with isoflurane and prepared for the measurement of heart rate (HR), mean arterial pressure (MAP) and the electroencephalogram (EEG). The inspired isoflurane concentration (Fi'_ISO) was then adjusted until the suppression ratio (SR) exceeded 20%, after which the corresponding end-expired concentration (Fe'_ISO) was maintained for 30 minutes. The effects of noxious stimuli, consisting of pinching (nasal septum, interdigital web, anus, periople) and clamping (tail, claw) on spectral edge frequency 95%, median frequency, alpha/delta ratio, beta/delta ratio, theta/delta ratio, total power, suppression ratio, MAP and HR were recorded. Fe'_ISO was then decreased by 0.3% and maintained for 30 minutes after which noxious stimulation was re-applied. This was repeated twice with Fe'_ISO reductions of 0.3% being applied each time. Changes in measured variables before and after noxious stimulation were then compared at each of four levels of anaesthesia. Results Noxious stimulation caused significant MAP increases at the lowest two levels for Fe'_ISO. There were no significant changes in the other variables examined. Conclusion Of the variables examined, mean arterial blood pressure is the most sensitive indicator of nociception in isoflurane-anaesthetized pigs. Clinical relevance When evaluating nociception under isoflurane anaesthesia in pigs mean arterial blood pressure should be monitored when surgery is performed.